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Full Article in PDF Format Estonian Journal of Earth Sciences, 2020, 69, 1, 2020–36–36 https://doi.org/10.3176/earth.2020.02 Muscle scars, mode of life and systematics of Pollicina (Mollusca) from the Ordovician of Baltica John S. Peel Department of Earth Sciences (Palaeobiology), Uppsala University, Villavägen 16, SE­75236 Uppsala, Sweden; [email protected] Received 12 August 2019, accepted 17 October 2019, available online 19 December 2019 Abstract. Pollicina is a distinctive, but uncommon, univalved mollusc originally described from the Middle Ordovician (Darriwilian Stage) of the Baltic. The slender, bilaterally symmetrical shell expands slowly and is curved through up to about 90 degrees, but straightens in the latest growth stages. Pollicina corniculum, the type species from the St Petersburg region of Russia is redescribed, as is Pollicina crassitesta, the most common representative from the Tallinn area of Estonia. Muscle scars in P. crassitesta form a continuous circum­apertural band on internal moulds about half way between the apex and the apertural margin. The bilaterally symmetrical shell, orthocline aperture, circum­apertural muscle scar and frequent displacement of the apertural margin, as evidenced by dislocations in growth ornamentation, indicate that Pollicina lived as a limpet, clamped against the substrate. Suggestions that it was an open coiled gastropod lying on the sediment surface are rejected. As with most Ordovician limpet­formed shells, assignment of Pollicina to Tergomya or Gastropoda is equivocal, even controversial, not least on account of the tall shell and muscle band. Despite similarities with the tergomyan Cyrtolites, Pollicina is placed tentatively together with the archinacelloidean gastropods. Key words: Mollusca, Gastropoda, muscle scars, habit, Ordovician, Darriwilian, Baltica. INTRODUCTION been suggested that the earliest gastropods were limpet­ formed (Golikov & Starobogatov 1975; Haszprunar 1988; Univalved molluscs with cap­shaped or limpet­formed shells Parkhaev 2017). However, Lindberg (1988, 2008, 2009) have existed throughout the Phanerozoic. Vermeij (2017) and Ponder & Lindberg (1997) proposed that the ancestor calculated that this shell morphology has arisen of patellogastropod limpets was likely a coiled gastropod. independently more than 50 times in the evolutionary history The oldest confirmed patellogastropod is Triassic in age, of gastropods, and to this may be added a morass of based on studies of shell structure (Hedegaard et al. 1997), monoplacophoran and poorly known bilaterally symmetrical although numerous claims of Palaeozoic patellids exist in shells in the early–middle Palaeozoic (Runnegar & Jell 1976; historical and more recent literature, as discussed below. Peel 1991a, 1991b; Geyer 1994; Parkhaev 2008, 2017; Thus, the early history of undoubted patellogastropods, or Bouchet et al. 2017). Most familiar at the present day are the their supposed coiled ancestors, has not been established patellids which dominate rocky shores at intermediate in the Palaeozoic, almost half of their supposed geological latitudes and which can be used to illuminate the dilemma range, frustrated by the inadequacy of shell­based facing malacologists seeking to unravel gastropod information of the almost featureless shells and confusion evolutionary history. Despite a host of modern phylogenetic with morphologically similar shells of co­existing and developmental studies (Koufopanou et al. 1999; monoplacophoran mollusc groups (Peel 1991a, 1991b; Wanninger et al. 1999, 2000; Lindberg 2009; Kristof et al. Yochelson & Webers 2006). Fortunately, ongoing studies 2016), elucidation of their palaeontological record is of protoconchs and new phylogenetic approaches (Frýda frustrated by the morphological simplicity of their bilaterally et al. 2009; Frýda 2012) offer a potential way forward. symmetrical shell and muscle scar, the only characters that This paper examines a distinctive but poorly known are preserved in most fossil scenarios. Ordovician univalve which lies in limbo between Lindberg (1986, 1988) proposed that Patellogastropoda gastropods and monoplacophoran molluscs. Pollicina has was the sister group of other gastropods, although this proven to be enigmatic ever since the name was introduced hypothesis has since evolved (Zapata et al. 2014). It has by Holzapfel (1895) in a publication which described © 2019 Author. This is an Open Access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International Licence (http://creativecommons.org/licenses/by/4.0). 20 J. S. Peel: Ordovician mollusc Pollicina Devonian fossils from Germany. However, the designated described from the East Baltic. This Ordovician type type species of Pollicina is an Ordovician form originally species, Pollicina corniculum (Eichwald, 1860; Fig. 1), has Fig. 1. Pollicina corniculum (Eichwald, 1860). Ordovician, Darriwilian, Kunda Stage, Popowka or Pulkowa [=Pulkovo], St Petersburg, Russia. A–F, lectotype, PSM 4/22: A, lateral view; B, supra­apical surface; C, lateral view; D, supra­apical surface at apex, with adherent matrix on right side; E, sub­apical surface; F, apertural view. G, H, replica of lectotype, PSM 4/22, showing prominent comarginal rugae: G, oblique lateral view; H, supra­apical surface. I, K, replica of paralectotype, PSM 4/23. J, CNIGRM 15692 (380/10903), sub­apical surface showing irregular longitudinal ridges. Arrow a locates equivalent points in B and H. Arrow b indicates equivalent points in C and G. Arrow c indicates the prominent transverse growth dislocation in A–C, H. Arrow r indicates ridges perpendicular to margin in I, J. Scale bar = 5 mm. 21 Estonian Journal of Earth Sciences, 2020, 69, 1, 20–36 been evaluated in several studies, (Knight 1941, 1952; ornamentation are lacking (Figs 2D–J, 3C–O), although Kisselev 1994; Evans & Cope 2003; Yochelson & Webers a rare exception is provided by the lectotype of Pollicina 2006) but the Devonian species proposed by Holzapfel crassitesta Koken, 1897 (Fig. 2A–C). This general lack (1895) has not. of information concerning ornamentation urges caution in The slowly expanding, slightly curved, bilaterally comparisons of internal moulds both with Pollicina symmetrical shell of Pollicina (Fig. 1) is universally corniculum and with internal moulds from other localities accepted as molluscan in origin but the position of the and horizons. The internal moulds are valuable, however, genus within Mollusca is controversial. Several of the in showing details of shell musculature that are not known various earlier interpretations as cephalopod, ‘hyolithid’ in Pollicina corniculum. pteropod, capulid gastropod, euomphaloidean gastropod, Pollicina is re­described on the basis of its two most monoplacophoran or the problematic xenoconchs and common species: Pollicina corniculum from the St toxeumorphorids were reviewed by Evans & Cope (2003) Petersburg region of Russia and Pollicina crassitesta who placed Pollicina within the monoplacophoran Class from Estonia. The latter material preserves muscle Tergomya Horný, 1965a sensu Peel 1991a. However, scars which promote a discussion of the systematic Yochelson & Webers (2006) maintained that Pollicina position of Pollicina. The distribution of shell was a gastropod. Bouchet et al. (2017, p. 194) noted that attachment muscles, together with the nature of growth usually Pollicina has been treated as a gastropod but that ornamentation in Pollicina corniculum, indicate that at this view had been rejected by Evans & Cope (2003). least some specimens of Pollicina lived as limpets The late Ellis L. Yochelson (1929–2006) nurtured an clamped against hard substrates. interest in Pollicina that extended over four decades and Repositories of figured specimens are indicated by the resulted in several unpublished manuscripts, aspects of following prefixes: CNIGRM, F. N. Chernyshev Central which were cited by Peel & Yochelson (1984), Evans & Geological Survey Research Museum, St Petersburg; Cope (2003) and Yochelson & Webers (2006). Yochelson ELM, Estonian Museum of Natural History, Tallinn; & Webers (2006, p. 25) summarized Yochelson’s theory PSM, Museum of the Department of Historical Geology, that Pollicina was an uncoiled or open coiled mollusc St Petersburg State University. related to euomphaloidean gastropods, an opinion that is not supported herein. However, Yochelson’s interest has materially advanced the present study since he passed on AUTHORSHIP OF POLLICINA to me photographs and replicas of the type material of Pollicina corniculum (Eichwald, 1860), the lectotype and Holzapfel (1895, pp. 182–183) attributed authorship of paralectotype of which are illustrated herein (Fig. 1). Pollicina to ‘Koken (in Litt.)’ quoting a written Yochelson’s photographs of the original specimens and communication from Ernst Koken in which the type was the replicas of these specimens were produced in designated as Cyrtolites laevis Eichwald from the Washington D.C. during the late 1980s and their identity Ordovician of Baltica. In this letter, Koken noted that he is confirmed by comparison with subsequent illustrations had used the name Pollicina in a manuscript, and it is of the types by Kisselev (1994). It is evident, however, reasonable to assume that this was the manuscript of that Yochelson’s photographs and the replicas were Koken (1897). Eichwald (1842, p. 71) had assigned prepared prior to the preparation of Kisselev’s (1994, Cyrtolites laeve to Cyrtoceras laeve [Sowerby in] fig. 1A–D) illustrations of the designated
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